Methods for diagnosing cardiovascular and related disorders can be classified into one of two categories: invasive diagnostic methods, and non-invasive diagnostic methods. Invasive cardiovascular diagnostic methods are those which involve penetration of the skin or a bodily orifice. Methods of this type include the insertion of catheters and the administration of radioactive pharmaceuticals into the body. Non-invasive cardiovascular diagnostic methods do not involve penetration of the skin or a bodily orifice. Methods of this type include the administration of physical examinations, electrocardiograms, and various forms of imaging examination.
Invasive cardiovascular diagnostic methods have gained wide spread use because they have been traditionally more reliable in diagnosing cardiovascular disorders than non-invasive cardiovascular diagnostic methods. However, non-invasive cardiovascular diagnostic methods are preferable because they are usually less expensive to administer and pose less risk to the patient than do invasive cardiovascular diagnostic methods.
One particularly effective non-invasive cardiovascular diagnostic technique involves palpating the radial arterial pulse with one's fingers. This technique has been successfully employed for centuries by practitioners of traditional Chinese Medicine and many medical practitioners in other cultures, including the Greeks, East Indians, Arabians and the British. In many of these cultures, where expensive diagnostic tools may not be readily available, palpation of the radial arterial pulse has been the first and sometimes only cardiovascular diagnostic tool. This tool is also effective in diagnosing other diseases since many disease states have concomitant cardiovascular manifestations.
Palpating the radial arterial pulse as a method of diagnosing disease, including cardiovascular disease, is based on the finding that the shape or contour of the arterial pulse contains important diagnostic information concerning the physiological state of the patient. When an arterial pulse is generated centrally at the heart, the pulse propagates into all parts of the circulatory system or vasculature, and is later reflected back centrally, like a wave reflected back by the edge of a pond. The magnitude and timing of the reflection or the contour of the reflection event depends critically on the state of the blood vessels which supply different organ systems in the body. One can thus think of the centrally generated arterial pulse as a interrogating test pulse, the reflection of which contains important diagnostic information concerning the physiological state of the vasculature and the end organs it supplies. As described in M. F. O'Rouke, R. Kelly and A. Avolio, The Arterial Pulse (Philadelphia: Lea & Febiger, 1992) , this reflected wave, together with the events associated with heart valve closures, plays a dominant role in the shaping of the arterial pulse contour.
However, the art of palpating the radial arterial pulse as a method of diagnosing cardiovascular and related diseases has not attained widespread acceptance. This may be attributed to the fact that the art does not yield quantitative results. As a result, reliable diagnoses can only be achieved by experienced practitioners of the art.
The blood pressure cuff or sphygmomanometer, which provides quantitative values of the systolic and diastolic blood pressures, has attained widespread acceptance as a method of diagnosing cardiovascular health. However, while the systolic and diastolic pressure values have been important in characterizing a patient's cardiovascular health, the systolic and diastolic pressure values do not include much of the important diagnostic information contained within the arterial pressure pulse contour. As a result, the use of sphygmomanometer as a means of diagnosing cardiovascular and related diseases is limited.
Systems have been developed to improve upon the method of diagnosing cardiovascular disease offered by the sphygmomanometer. For example, U.S. Pat. No. 5,638,823 to Akay et al. teaches a system for non-invasively detecting coronary artery disease. The system performs a wavelet transform on an acoustic signal representative of the diastolic heart sounds of the patient. The first four moments of the wavelet transform are included in a feature vector which is input into a neural network. The neural network diagnoses the presence of coronary artery disease based on the information contained in the feature vector.
However, the system taught by Akay is deficient in several respects. First, since the system analyses the acoustic turbulence associated with blood flowing in the patient's arteries, the system is capable of detecting the presence of one disease only, namely coronary arterial stenosis. Second, the system requires that the patient be administered a vasodilator drug to improve the signal-to-noise ratio of the diastolic heart sound. Therefore, the system is limited by the availability of such drugs and the allergic profile of the patient. Third, many clinical parameters, such as the patient's sex, age, weight, blood pressure and family medical history, must be included in the feature vector. As a result, the possibility of mis-diagnosis is dependent upon the accuracy of the clinical parameters input. Lastly, the system is highly susceptible to mis-diagnosis through contamination of the diastolic heart sound through other sounds such as ambient, respiratory and stomach noise.
U.S. Pat. No. 5,533,511 which issued to Kaspari teaches a method and apparatus for the non-invasive measurement of blood pressure. The method involves obtaining blood pressure pulses from a patient's blood pressure waveform by placing a piezo-electric sensor over an artery. The output pulses of the sensor are then filtered, amplified, and digitized. Time features (such as pulse amplitude, rise time, and pulse width) and frequency features (such as Fourier or Laplace transform values, phase relationships and frequency distribution) are extracted from the digitized pulses. Finally, a neural network determines the actual blood pressure of the patient based on a comparison between the extracted features and historical data acquired using blood pressure data from a plurality of patients.
However, the method taught by Kaspari is limited only to the estimation of the blood pressure pulse for clinical monitoring, and due to the nature of the Kaspari method, it is not readily adapted for the diagnosis of cardiovascular disease. Furthermore, according to the method taught by Kaspari invasive intra-arterial catheters must be used to obtain reference data of absolute blood pressure. Invasive procedures such as required by Kaspari will not be acceptable to all patients and may even pose a risk to some patients.
Accordingly, there remains a need for a method of rapidly, accurately and non-invasively diagnosing cardiovascular and related diseases.